This invention relates generally to method of and apparatus for controlling liquid level and power consumption in liquid treatment systems wherein surface aeration devices are employed in enclosed covered chambers to promote the dissolution of a gas into a liquid.
In the practice of oxygenation of BOD-containing water as taught by U.S. Pat. Nos. 3,547,812-3,547,815 to J. R. McWhirter et al, at least one enclosed covered aeration chamber is employed wherein the liquid undergoing treatment is intimately contacted in the presence of activated sludge with oxygen-enriched gas from an overlying gas space to dissolve the oxygen necessary for aerobic biological activity. Such oxygenation systems provide substantial advantages over prior art treatment systems wherein atmospheric air is used as the oxidant in open aeration chambers. For example, the closed chamber oxygenation system is able to operate at biological suspended solids levels several times greater and aeration detention periods several times less than those of air aeration systems while maintaining comparable or better overall levels of treatment. Such advantages are a consequence of the higher mass transfer driving force for oxygen-enriched gas relative to air, which permits higher dissolved oxygen levels to be achieved with economic levels of volumetric oxygen transfer rate per unit of power input. A particularly effective method for obtaining high economic utilization of oxygen gas in the aeration feed gas is disclosed in U.S. Pat. No. 3,547,812, wherein the feed gas comprises at least 60% oxygen (by volume) and the oxygen feed gas to total (mixing plus gas-liquor contact) energy ratio is maintained at 0.03-0.40 lb. moles oxygen per horsepower hour of energy supplied. In order to suitably employ such feed gas-energy ratios, it is necessary that the energy supplied to the aeration zone be used efficiently to generate the gas-liquor interfacial area required for oxygen solution. Mixing energy must also be employed to hold the solids uniformly in suspension and to circulate the mixed liquor repeatedly through the gas-liquor contactor. For these reasons, it is taught by the aforementioned patent to use an aeration device whose standard air transfer efficiency is at least 1.5 and preferably 2.5 lb. oxygen/horsepower hour in order that the oxygen may be dissolved rapidly despite the relatively small volume of gas fed to the system, and in order that the biological solids are not damaged and dispersed.
In practice surface aeration devices are often used in the aforementioned oxygenation systems to achieve the desired levels of gas-liquid contacting. These devices are disposed at or near the surface of the liquid to be treated and project relatively large sheets or streams of liquid into the gas phase overlying the liquid pool, thereby providing extensive interfacial area for mass transfer while promoting circulation and fluid mixing in the bulk liquid volume. Surface aerators may be of various types, as for example relatively low speed rotating impellers, disks, or brush devices or high speed propellers mounted in draft tubes. Such devices can provide high rates of oxygen transfer, e.g. 2-4 lb. oxygen/hour per unit of total horsepower input, and thus represent highly efficient means for effecting gas/liquid contact. In contrast to other aeration devices such as diffusers and submerged turbine rotating spargers, surface aerators are not susceptible to clogging by solids, are relatively simple mechanically and do not require gas recirculation compressors and associated piping.
In spite of their desirable features, however, certain disadvantages have become apparent as surface aeration devices have been employed for the treatment of BOD-containing water in closed oxygenation chambers. Surface aeration units are inherently sensitive to changes in liquid submergence, which can greatly affect their efficiency, power requirements and stability of operation. Operation at too shallow an aerator submergence provides insufficient oxygen dissolution for the process requirements and may result in mechanically unstable operation which can drastically shorten equipment life. Alternatively, too great a submergence may result in deleterious overloading of the drive means, e.g., burnout of an electrical motor, and necessitate the shutdown of the treatment system. In addition, operation at too high or too low a submergence will frequently result in cyclic projected liquid patterns and wave motion at the liquid surface, accompanied by severe power surges. For these reasons, the range of tolerable submergence levels for a given surface aeration device may be quite narrow, as for example 2-3 inches overall. Nonetheless, liquid level changes due to pumping cycles within the liquid treatment facility or variations in the influent hydraulic loading may result in liquid submergences which are outside of the desired submergence limits by a factor of two or more in a system lacking means for liquid level control. Under such conditions, the increase in power draw from low submergence to high submergence levels may be substantial, e.g., greater than 60%.
Heretofore, closed chamber oxygenation systems have generally employed a respirometer method of oxygen feed control whereby the feed gas flow control means are responsive to a slight, predetermined superatmospheric pressure in the gas space of the oxygenation chamber. In practice, a simple pressure sensor is positioned in the aeration chamber cover to detect changes in gas space pressure resulting from a decrease or increase in oxygen uptake, as flow of or strength of the influent liquid changes. A control signal based on the pressure sensing is then generated and relayed to a flow control valve on the inlet aeration gas line which adjusts gas flow to maintain the desired set point gas pressure in the oxygenation chamber. Thus, if process changes result in a lowering of gas pressure in the chamber gas space, for example, the control valve will open causing more oxygen-enriched gas to flow to the system. This simple control circuit provides steady, uniform feed of oxygen gas and is usually responsive solely to the oxygen demand and/or mass transfer capability of the system. The selected superatmospheric pressure which it maintains within the oxygenated chamber is only sufficient to reject spent gas from the chamber and is dependent upon the flow resistance of the gas venting means which in turn are usually adjusted to optimize oxygen utilization in the chamber.
Despite its comparatively rapid response to changes in oxygen demand placed on the oxygenation system, however, the above described respirometer system achieves no direct control over liquid level in the closed chamber. In other words, a given gas pressure setting for the oxygenation chamber can be maintained regardless of the liquid level therein. If the influent flow rate of liquid to the oxygenation system varies, then the liquid level in the oxygenation chamber will change correspondingly with the extent of liquid level change depending upon the type of liquid discharge means used to control liquid inventory in the system. In practice, the liquid discharge means generally include a discharge weir which may be disposed external of the oxygenation chamber as for example in a clarification basin downstream from the chamber or alternatively may be positioned in the oxygenation chamber.
If a weir is employed externally of the covered chamber to control liquid level therein and if the oxygen supply to the chamber is regulated in accordance with the respirometer method, then variations in gas pressure within the closed chamber, caused for example by altering the flow resistance of the gas venting means, can produce severe changes in liquid level, with the aforedescribed adverse effects on surface aeration devices positioned in the oxygenation chamber. The severity of the liquid level changes under such conditions is due to the fact that external weir system behaves similar to a manometer in that an increase in gas space pressure displaces liquid from the oxygenation chamber until balance is restored between the respective hydrostatic pressures inside and outside of the chamber.
A further problem encountered with the combination of respirometer control and external weir liquid discharge in the oxygenation system is that liquid level changes therein due to hydraulic load variations tend to be reinforced rather than minimized by the system response. A rise or fall in the liquid level in the oxygenation chamber tends to correspondingly compress or expand the body of gas overlying the liquid. Absent the respirometer-controlled feed gas flow control valve, variations in liquid level attendant hydraulic load changes would be dampened by the "cushioning" effect of the gas volume in the chamber. However, the respirometer-controlled valve reacts in such a way as to eliminate the dampening effect of the gas thereby creating submergence conditions potentially detrimental to surface aeration devices disposed in the oxygenation chamber. For example: when hydraulic load on the treatment system is suddenly decreased, the liquid level in the oxygenation chamber will tend to drop. With such occurrence, the pressure in the gas space will drop due to rapidly increasing volume of the gas space. Pressure will fall below the set point pressure, as detected by the pressure sensor. The respirometer system will then send a control signal to further open the feed gas supply valve, to feed more oxygen-enriched gas into the chamber and thereby increase the gas space pressure. In this manner, the control system tends to depress the liquid level so that surface aerator submergence may remain too shallow. When the hydraulic load on the oxygenation system is suddenly increased, the liquid level in the chamber rises and the gas space volume is correspondingly decreased by compression, so that the gas space pressure increases. Such pressure increase is sensed by the control means, which then act to reduce the gas space pressure by further closing the oxygen aeration gas supply valve. This control system thus may tend to maintain the surface aerator at an excessive submergence under such conditions, with the aforementioned deleterious consequences to the aeration apparatus.
In the other past applications of the closed chamber oxygenation systems where the discharge weir is positioned internally of the chamber, the liquid level and liquid discharge rate are relatively insensitive to variations in gas pressure in the chamber, regardless of the type of oxygen gas feed control. However, with such provision, the weir is inaccessible in the closed chamber and manual adjustment thereof is both inconvenient and expensive. Alternatively, the use of automatic weir adjustment means would require internal mechanisms and positive sealing of the closed chamber and would result in a system which is expensive, mechanically complex and difficult to maintain without system shutdown.
Moreover, it is frequently difficult to accommodate a weir within the confines of the chamber which is sufficiently long and extended to minimize the effects of variations in hydraulic loads. Operationally, the use of an internal weir introduces an additional hydraulic loss which would not otherwise be present and which must be accomodated by provision of additional pumps or pumping capacity in the treatment system.
In connection with the above, it is often desirable to adjust the submergence of the surface aerator device within fairly narrow limits to vary the level of gas-liquid contacting and power input to the aerator, as for example in response to cyclic changes in the oxygen demand of the liquid undergoing treatment. A fixed position internal weir cannot accomodate a wide range of oxygen requirements in the liquid undergoing treatment and maintain an optimium submergence and aerator power input under all conditions. With a fixed position internal weir, the power input to the aerator may be varied by changing the speed of rotation of the aerator, but this requires variable speed drive mechanisms which are expensive to provide and maintain and which usually represent an added inefficiency in the power transmission system. The capability to reduce consumption during periods when oxygen demand is low is important for economy and for energy conservation in the oxygenation system. Similarly, the ability to increase power input therein to meet cyclically- or seasonally-high oxygen demand is important in order to avoid inadequate treatment of wastewater and consequent pollution of receiving waters.
Another important disadvantage associated with the inflexibility of the fixed internal weir is the difficulty in accomodating permanent, progressive increases in liquid flow rate. It is common practice to overdesign a treatment plant in deference to anticipated long-range load increases. For example, a plant may go onstream at half its design capacity, with the full (design) load not expected until a number of years later. Under such conditions, it is often desirable to operate the water treatment facility at a considerably lower oxygen dissolution capability than the ultimate plant load will require. Such provision makes it necessary to operate initially at a lower aerator submergence which is compatable with the relatively low initial dissolution capability and later at an increased aerator submergence for higher dissolution capability. As through put increases, a plant with internal weirs must be periodically modified, either to reposition weirs and/or to adjust the elevation of the surface aeration devices. Such changes are expensive, time-consuming and require shutdown of the facility.
Accordingly, it is an object of the present invention to control liquid level in an enclosed chamber contacting system employing surface aeration in such manner as to continuously minimize liquid fluctuations which would deleteriously affect the surface aeration apparatus.
It is a further object of the invention to control gas space pressure level in an enclosed chamber contacting system employing surface aeration in such manner as to continuously minimize liquid level fluctuations and to limit the aeration power expenditure.
It is a still further object of the invention to provide means for controlling submergence of surface aeration apparatus in enclosed covered aeration zones which are readily adjusted to accommodate changes in both the oxygen requirement of the liquid undergoing treatment and in the hydraulic loading of the aeration zone.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.